1. Field of the Invention
This invention relates to a data transmission network. More particularly, the invention relates to measuring and correcting a token ring for ring latency.
2. Description of the Prior Art
In token rings, "ring latency" is defined as the time required for a token to circulate on the ring when the network is idle. It is important that a network have the capability of measuring ring latency as ring latency changes each time the token circulates the ring. The frequency and amplitude of the change in latency is called "jitter". Typically, the reason "jitter" occurs is due to synchronization or clock problems, hardware problems, too long a wire in between adjacent station, etc. It is important to measure "jitter" to permit a monitoring station on the token ring to compensate for it by putting extra bits on the ring or taking bits off the ring. By measuring latency for each token circulating the ring, a prediction can be made as to whether or not latency is increasing or decreasing. If latency is increasing, it will cause performance problems or even bring down the network. Knowing this condition is increasing will allow the time to find out what is causing the problem and return the ring to the normal state before a failure occurs.
In the prior art, an IBM TDB, April 1990, page 262, provides a means for measuring ring latency of a token ring by timing total number of bytes in a message as a calculation of latency. This disclosure does not measure latency frequently enough to detect trends and allow action before a problem is apparent to a ring user.
U.S. Pat. No. 5,206,857 discloses a means for measuring the ring latency by using a counter to count the time the number of cycles for a node to receive its own packet which has traversed the entire ring. A clock is used to provide timing information so that the propagation delay or ring latency measurement can be measured by a master node with a clock. Again, the disclosure does not measure latency frequently enough to detect trends and allow action before a problem is apparent to a ring user.
U.S. Pat. No. 5,235,593 describes a timer for measuring the ring latency of a token ring at each node. The '593 patent does not do anything to measure minimum or maximum latency or the number of latency changes as a basis for determining ring latency caused problems and latency trends.
U.S. Pat. No. 5,313,466 discloses a method for real time message transmission in terms of latency in sending messages with settings on maximum transmission time. The '466 patent does not provide a mechanism for "jitter" control as a means of keeping the network in a stable condition.
U.S. Pat. No. 5,247,626 discloses a means for configuring an FDDI controller to accommodate a particular average bus latency and not the latency on a token ring.
IBM TDB, February 1994, pages 349-350 discloses measuring latency and bandwidth separately and independently of a communication link type.
U.S. Pat. No. 5,206,857 discloses a technique for distributing timing synchronization among the selected nodes and not the measurement of ring latency.
U.S. Pat. No. 4,663,748 discusses latency in terms of importance to traffic with respect to providing two ways of transmitting messages on a short cycle or a long cycle such that control can be exercised whether its a high priority or low priority traffic from the cycle period.
U.S. Pat. No. 5,309,428 discloses putting data on a ring and measuring with a clock the amount of time a frame transmits on the ring. The '428 patent does not utilize a token revolving naturally around the ring and the occurring frame transmissions for controlling latency.
None of the prior art measures latency on a ring without a clock. Nor does the prior art measure latency on a ring by counting bits between successive tokens or a token and the following frame to determine the amplitude and frequency of latency. Further, the prior art does not passively determine latency on the ring or have the ability to detect the insertion and removal of a station on the ring. The ability to passively measure latency on a ring without the use of clocks will advance the measurement and control of high speed data networks which are particularly sensitive to failure due to latency.